Although Windows 10 officially launched yesterday, many are complaining that they haven't yet got the upgrade. Even if they pre-registered for the download at the beginning of June, they are still plugging away with Windows 8.1 or 7, hoping that a notification will pop up to tell them their PC is ready to install the new operating system.

Sadly, that might take a few days yet, as Microsoft is sending out the upgrade in "waves". That will ensure that its servers aren't completely overrun with requests.

So if you are wondering why your upgrade hasn't arrived yet, or something else about Windows 10, let us explain below...

Microsoft opened for pre-registration by those interested in downloading Windows 10 on day one at the beginning of June, but it soon became apparent that the demand was so great that it was not going to be possible to serve the software to everybody on launch day.

Indeed, the company revealed that the only people likely to get Windows 10 on day one were developers - members of the Windows Insider programme.

READ: Windows 10 won’t be available to everybody at launch

The rest of the Windows 7 and 8.1 owners that are eligible for a free upgrade would have to wait a bit longer - anywhere up to a week longer, in fact.

If you pre-registered, you are in a queue depending on when you requested your upgrade, so it will be served when it is your turn. If you are yet to register for the upgrade, sadly you will be one of the last to get it.

READ: How to get your free Windows 10 upgrade

There are several key changes and features in Windows 10 that weren't in previous iterations of the operating system. Microsoft's new browser, Edge, the Cortana personal assistant, and the return of the start menu are just three.

You can read about the key features in our feature, 10 new features you’ll love in Windows 10, or check out our first impressions test to see what we think of the consumer build.

Microsoft Edge is the web browser that replaces Internet Explorer and it comes free with Windows 10. You can read about some of the key changes between the two in our guide, 9 things you can do with Microsoft Edge you couldn’t with Internet Explorer.

Considering that Windows 10 is a free upgrade for Windows 7 as well as Windows 8.1, it is capable of running on some pretty old PCs. In fact, it is faster and has a smaller footprint than former versions so might even improve the performance of older machines.

Microsoft itself claims that it will require at least a 1GHz processor, 1GB of RAM (2GB for 64-bit machines), 16GB of HDD storage space for 32-bit machines/20GB for 64-bit, DirectX 9 or later (with WDDM 1.0 driver), and a minimum display resolution of 800 x 600.

If you are looking to upgrade your PC along with the software, we've come up with a list of laptops we'd recommend that will work brilliantly with Windows 10 - 5 best laptops for Windows 10: The best available to buy today.

Microsoft claims that the upgrade will only take around 20 minutes to install after the download has completed. If you pre-registered, the download should happen automatically and might even be ready for you without you noticing (overnight even, if you have your PC on when you go to bed).

As for how long it takes to download, it depends on your broadband connection. Microsoft claims that the file is around 3GB in size so can take over an hour and a half on a 4Mbps connection, or around four minutes on 100Mbps broadband. In real life reports, the file is more like 6GB so you can double those times.

We've written a few tips and tricks on how to change certain things in the settings and manipulate other features in our round-up - Windows 10 tips and tricks: Here's what your PC or tablet can do now. You should check that out and also see our guide on some of the things we feel you should do the moment you get to play with the software for the first time - Downloaded Windows 10? Make sure you do these five things first.

READ: 9 things about Windows 10 you didn’t know already

There are lots of other things you'll probably want to know about Windows 10, like how Cortana works or how you can link your Xbox One to a PC and stream games across a home network, but much of the joy in getting a new user interface is in exploring it for yourself.

If you have any other questions you'd like us to look into, please don't hesitate to ask in the comments below. Or let us know if you've found any interesting secrets or features.